1. Field of the Invention
The present invention relates to a receiving apparatus of a radio communication system and, more particularly, to a diversity receiving apparatus.
2. Background of the Related Art
A diversity technique is used to improve a quality of a received signal in an analog or a digital radio communication system.
The diversity system generally includes two or more antennas and signals received through each antenna are suitably selected, switched or combined, so that a better quality of a received signal is obtained compared to a receiving device which uses only one antenna.
In a high data rate (HDR) system, one of the third-generation synchronous mobile communication standards in service, the diversity technique is highly recommended for the receiving unit of a terminal.
The receiving unit of the HDR terminal adopts an adaptive modulation method according to a received signal quality. Generally, the better the quality of a received signal a terminal has, the faster it can receive data. Thus, influence of a receiving performance of the terminal on an overall system is even stronger compared to existing second-generation mobile communication systems.
Diversity techniques include a selection or switching technique, an in-phase combining or equal-gain combining technique, and a maximal ratio combining technique.
The selection technique operates as to selectively receive the strongest signal among signals received by a plurality of antennas. Thus, as long as all signals received by the plurality of antennas do not simultaneously drop down to below a reception sensitivity, a reception signal quality above a certain level can be obtained.
In order to use the selection technique, a controller of a receiving side should be aware of a strength of each signal received by each antenna. In the case where the selection technique can not be used, the switching technique can be used instead.
The switching technique operates as to switch to a different antenna (for example, a second or a third antenna) if a strength of a reception signal currently being received by one antenna (a first antenna) drops to below a threshold value. With the switching technique, quality of a reception signal can be improved in a similar fashion to that of the selection technique.
Implementation of the selection or the switching techniques can result in improvement of the signal quality in a fading environment where the strength of the reception signal changes over time, however, no improvement is realized in a static environment where a strength of the reception signal is not changed.
The in-phase combining technique operates such that of the in-phase components of the signals received from each antenna are combined and the combined signal is used as a final receiving signal. By not simply selecting a signal but combining signals, the in-phase combining technique ensures an improvement of a signal quality greater than the selection or switching technique.
The in-phase combining technique requires a phase shifter for controlling a phase of a signal but can improve a received signal quality even in a static environment as well as in a fading environment.
The signals to be combined may have a big power difference, however, both signals contain a similar amount of noise component. Thus, combining the signals results in obtaining a small amount of gain and double the noise component, causing the signal-to-noise ratio of the combined signal to deteriorate.
A maximum ratio combining technique operates such that both the individual phases of received signals and the signal gains are controlled and then combined.
In the maximum ratio combining technique, when signals are received by each antenna, the powers of the received signals are detected, and the signal with greater power is more amplified while the smaller power signal is attenuated. The two signals S1 and S2 are then combined. The reason for amplifying (or attenuating) the signals prior to combining is to avoid problems of the in-phase combining technique as described above.
Accordingly, when a signal with less power is attenuated, the noise component is also attenuated and signal-to-noise ratio degradation of the combined signal can be prevented.
In spite of the advantages of having a great effect on improving signal quality, the maximum ratio combining technique is disadvantageous in that such a device is too complicate to be efficiently implemented and a large number of complicate calculations need to be made in comparison to other diversity techniques.
FIG. 1 illustrates a construction of a diversity receiving apparatus in accordance with the related art. This apparatus includes a phase shifter 10 for shifting a phase of a signal (first signal) received through a first antenna by 0°, 90°, 180° or 270°. A signal combiner 20 combines the phase-shifted first signal with a signal (second signal) received through a second antenna. A tuner 30 improves a signal-to-noise ratio of an output signal of the signal combiner 20 and prevents leakage of a radio wave. An amplifier 40 for amplifies an output signal of the tuner 30, and a frequency converter 50 converts a signal outputted from the amplifier 40 into an intermediate frequency (IF) signal An intermediate frequency amplifier 60 amplifies a signal of a certain frequency band centering around an intermediate frequency. A receiving electric field strength detector detects a strength of a signal outputted from the intermediate frequency amplifier, and a controller 80 controls the phase shifter 10 such that the detected strength of signal is optimized.
As the signal (the first signal) received through the first antenna passes through the phase shifter 10, it is phase-shifted by 0°, 90°, 180° or 270° and then transmitted to the signal combiner 20.
The signal combiner 20 in-phase combines the phase-shifted first signal with a signal (the second signal) received through the second antenna.
The combined signal sequentially passes through the tuner 30, the amplifier 40, the frequency converter 50 and then the intermediate frequency amplifier 60 so as to be down-converted into an IF signal.
The receiving signal strength detector 70 detects a strength of the combined signal which has passed through the intermediate frequency amplifier 60, and the controller 80 periodically shifts a phase of the phase shifter 10 so as to obtain the most powerful signal as measured by the receiving signal strength detector.
When the first and second signals are combined in a state in which they do not accurately have a phase difference of 0°, 90°, 180° or 270°, a combining loss is generated. This kind of signal combining is not considered, in the strict sense, an in-phase combining. However, the combining loss in this case is relatively insignificant.
The diversity receiving apparatus described above has several problems with respect to the in-phase combining technique.
First, in the case that the first and second signals have great differences in their power, a signal-to-noise ratio of the combined signal outputted from the signal combiner 20 is rather deteriorated.
Also, the loss of signal generated in the phase shifter and the signal combiner 20 increases an overall noise figure of a receiving apparatus. This degrades a receive sensitivity, resulting in reduced improvement receiving signal quality.
If the signal loss generated in the phase shifter 10 depends on a phase variation amount, when a phase of a received signal is shifted, the size of the received signal is also changed. Actually, because the phase shifter continuously performs the phase shifting, it generates a noise in the received signal and degrades signal quality.
In addition, because the signal loss due to the phase shifter 10 occurs only in the signal (first signal) received by the first antenna, an imbalance of an average power of the first signal and the second signal causes degredation of the diversity reception performance.